The broad aim of this proposal is to examine the adolescent circadian (~24-hour) system and its phase shifting responses to light in comparison to adults. Adolescents often report late bedtimes and a tendency to be most alert in the evening, behaviors that are partly driven by the endogenous circadian clock. The circadian cue for sleep and wake initiation is late during the teen years. Unfortunately, this late sleep pattern coincides with the early start time of high school. Thus, many adolescents in high school (aged 14-17 years) are faced with a conflicting biological drive to fall asleep late and a social drive to wake early for school. The resulting sleep restriction is associated with a number of negative outcomes, including poor academic grades, an increased risk for depressed mood, and decrements in daytime alertness. Bright light is able to phase shift (reset) the circadian clock, and thus could help youngsters adjust to their early school sleep/wake schedule. Commercially available light boxes could be used at home for treatment; however, it is essential to have a phase response curve (PRC) to light to know the optimal times of bright light administration and avoidance to produce the maximum phase shift in the desired direction. PRCs graphically illustrate the phase shifting response to a stimulus (e.g., bright light) when it is presented at different times across the 24-hour day. Currently, there are several PRCs to light for young adult humans, but no light PRC exists for adolescent humans. It remains untested whether the adult light PRC is appropriate to guide light treatment in adolescents. We aim to generate a PRC to light for adolescents aged 14-17 years and a PRC to light for adults aged 35 to 45 years. By doing so, we will examine the hypothesis that adolescents manifest smaller phase advance responses and/or larger phase delay responses to light compared to adults. We will also examine the hypothesis that adolescents manifest a longer free-running circadian period compared to adults. Participants will complete two counterbalanced 5-day laboratory sessions. Each session begins and ends with salivary melatonin sampling to determine the phase shift of the circadian clock. In between, participants keep an ultradian light/dark schedule (sleep for 2 h and wake for 2 h) for 3 days. In one session, no stimulus is given and the circadian system free-runs yielding the endogenous circadian period. In the other, a bright light stimulus is turned on once per day at various points of the circadian cycle. Resulting phase shifts after bright light ar corrected for an individual's free-running period, and are plotted relative to light administration time to generate the PRCs. This study may further our understanding of circadian factors associated with late sleep patterns during adolescence. Furthermore, PRCs to light generated from this study will have direct and practical applications for phase shifting circadian rhythms and treating circadian-based sleep disorders in these age groups.